Factors in the formation of the giant Kalgoorlie gold deposit

Kalgoorlie (Golden Mile plus Mt Charlotte) accounts for more than half the gold won from the 2000 deposits that comprise the widespread gold mineralization of the Archaean Yilgarn Block of Western Australia. It is also one of the largest gold deposits globally (> 1800 t Au production and reserves).An analysis of the various stages of formation of the Kalgoorlie deposits from source, through solution of gold in hydrothermal fluids and their infiltration along channelways, through to gold precipitation at depositional sites reveals several features that make Kalgoorlie favourably disposed as a site for a major gold deposit. However, none alone account for its anomalously large size.It is concluded that the giant Kalgoorlie gold deposit owes its size and unique position in Western Australian gold production history to favourable host-rock chemical composition, favourable host-rock thickness and mechanical properties, and to the favourable geometry of its host units and hosting greenstone belt, at favourable PT conditions within the greenschist facies. Collectively, these factors, together with proximity to the regional-scale Boulder-Lefroy fault, allowed highly focussed fluid flow on both a deposit and district scale. In this regard, it is significant that the structural setting of gold mineralization and the greenstone architecture at the largest Canadian Archaean deposit (Timmins) are remarkably similar to Kalgoorlie.Orebody size-distribution histograms for both the Yilgarn Block and the Abitibi Belt, Canada, are highly skewed, with extremely few giant deposits, but numerous deposits of less than or equal to 10 tonnes gold existing, This topology can be accounted for in terms of system efficiency. The probability of all six factors in the ore-forming system being optimized (high heat flow, large fluid reservoir, proximity to major structure, efficient fluid focussing, and large and compositionally favourable host lithology) is extremely small. In contrast, the probability of two processes coupling to give low efficiency is very large, resulting in numerous small deposits.Kalgoorlie appears to have formed by the same basic processes that formed the plethora of smaller Yilgarn gold deposits. However, it represents a mineralized domain in which there was a conjunction of a number of favourable parameters, both on a regional and deposit scale, that produced high system efficiency and allowed the effective fluid focussing and gold deposition that formed this giant deposit.